332 Proceedings of the Royal Irish Academy. 



phyt., p. 84. This last synonym is given on the authority of Heiberg, 

 who had the opportunity of inspecting authentic specimens. 



Cork Harbour, Wm. Smith. Rock-pool, Salt Hill, Co. DubHn. 

 Coast of Galway, from collections by jyL'Calla, in the Herbarium, 

 Trinity College, Dublin. 



Genus Y. Scnizo:N"EirA, Agardh. 



Frond usually much divided. Frustules arranged in one or more 

 files within the gelatinous tubes which constitute the frond. 



Smith remarks justly that " the fronds in this extensive genus 

 were amongst the earliest Diatomaceous organisms recognised by 

 naturalists, and have been the perplexity of all subsequent observers." 

 !N"or is this to be wondered at, when the difficulties attendant on their 

 examination are taken into consideration. The frustules enveloped 

 in the fronds are generally minute, so that even though they were 

 free it woxtM be no easy matter to examine them satisfactorily, and 

 the difficulty is much enhanced by the intervention of the fronds as 

 well as by the manner in which the frustules are packed within them. 

 Kiitzing attempted to arrange the species on the basis of the charac- 

 ters of the fronds, but with how little success the student will be 

 convinced who endeavours to make himself master of the subject by 

 the aid of his minute descriptions, and of his very indefinite figures. 

 Heiberg falls into a mistake the very opposite to that of Kiitzing — 

 discarding from consideration not only the characters of the fronds in 

 the various species, but regarding the fact of the frustules being normally 

 incased within fronds as an unreliable generic distinction, and so ho 

 ranks the species under the genus JSTavicula. De Brebisson had ob- 

 served that "the greater part of the species needed reconsideration, 

 and to be studied with regard to the character of the frustules." and 

 Smith, with his characteristic sagacity, taking this hint, at the same 

 time not overlooking any reliable character exhibited by the fronds, 

 dispelled the confusion which had hitherto existed, and reduced the 

 species into an order, which seems, all the circumstances considered, to 

 admit of little improvement. 



As to the mode of reproducing the sporangia in the genus, opinions 

 differ, as the following extract from Pfitzer will sufficiently show : — 

 "In Schizonema Grevillii, according to Smith, a single mother-cell 

 produces a single auxospore, while according to Liiders this occurs 

 but seldom, namely, when one auxospore becomes defunct ; usually, on 

 the contrary, two cells co-operate, and form two auxospores. The 

 development of the latter occurs outside the tubes in a large and fine 

 mucous investment. The mode of proceeding, according to Liiders, is 

 that each mother-cell divides itself, and the halves unite in pairs. 

 However, in other forms in which, according to Liiders, a similar 



